1. Field of the Invention
The present invention relates to chloride-selective electrodes. More particularly, the present invention relates to chloride-selective electrodes comprising insoluble metal salt layer and a protecting membrane formed of hydrophilic polyurethane thereon. The hydrophilic polyurethane coated chloride-selective electrodes show fast activation and response time and are usefully employed to accurately measure the chloride by reducing the interference from bromide and iodide or preventing the surface of the electrode from protein adsorption.
2. Description of the Prior Art
The level of chloride in a liquid is a useful index with which the liquid""s state can be perceived and thus, it is very important to fast and accurately measure the chloride levels in physiological fluids such as blood, urine and the like, in domestic water such as tap water and sewage, and in industrial intermediate and final products and by-products, for the purpose of clinical analysis, water examination and product quality analysis, respectively. Particularly, the chloride in serum and whole blood, which compose the majority of extracellular fluid anions, plays an important role in maintaining the osmotic pressure of blood to contribute to the control in blood amount and pressure. Accurate quantitative analysis of the chloride level in blood may yield a vast amount of clinically useful information.
There are reported several quantitative methods for determining chloride levels in blood: mercurimetric titration, argentimetric coulometry, spectrophotometry and potentiometry. The first three methods suffer from serious problems not only of using expensive reagents, but of being subject to a large variation in the analytical results depending on the technicians. What is worse, they are indirect methods requiring isolation of blood cells from blood serum, as makes it difficult to apply them for automated analysis or multisample analysis in clinical laboratories.
In contrast, the potentiometry is a direct method that doesn""t need such a pretreatment of samples and has apparent advantages over the previous methods in that it is relatively simple in analysis equipment and process, short in analysis time, and inexpensive in analysis cost. Additionally, it is independent of such typical barrier factors as turbidity of solution, hemolysis, bilirubin, etc.
Two types of ion-selective electrodes have been used for the potentiometric determination of blood chloride: solvent polymeric membrane-based electrodes and solid-state membrane-based electrodes.
Typically, a solvent polymeric membrane-based electrode is composed of a polymer such as polyvinyl chloride), polyurethane and silicone rubber, a chloride-selective compound such as metalloporphyrins, quaternary ammonium salts, and organomercuric compounds, and a plasticizer to provide a liquid-like state for the ion-selective material incorporated in the membrane (Anal.
Chim. Acta 1998, 367, 175-181). A significant disadvantage of this solvent polymeric membrane-based electrode is that it is quite susceptible to interfering lipophilic anions such as thiocyanate (SCNxe2x88x92) and salicylate, resulting in overestimation.
Another type of a solid-state membrane-based electrode, which uses as an ion-selective electrode membrane an insoluble metal salt layer such as AgCl and Hg2Cl2, has been typical chloride-selective electrode since the early days of research and it has been reported to be applied in clinical analysis. The solid-state membrane-based electrode can be easily miniaturized because of no internal reference solution and is readily incorporated in any potentiometric analyzers in various formats (e.g., wire, pellet, screen-printed electrodes, tubular electrodes, microchips, etc). In addition, its ability to be readily renewable by simply polishing their deformed or polluted surfaces allows for the provision of stable potentiometric responses for an extended period.
However, its application in clinical analysis has been limited because it suffers not only from a poor discrimination of anions (e.g., bromide and iodide) that form less soluble metal salts than AgCl or Hg2Cl2, but also from protein adsorption to the electrode surface.
Therefore, in order to effectively use the solid-state membrane-based electrode in clinical analysis, its blood and biocompatibility is required to be improved with reduction in the interference from bromide and iodide. In this regard, it has been suggested that cellulose acetate, poly(acrylic acid), or poly(methacrylic acid) be used as a protective coating on a solid-state membrane electrode (Anal. Chem. 1981, 53, 1164-1170; U.S. Pat. No. 4,199,412, 1980).
It is reported that these protective coatings effectively reduce the interference from bromide and uric acid. Another report gives the data which demonstrate that the biocompatibility of amperometeric sensors is greatly enhanced by modifying their surfaces with cellulose acetate.
The chloride-selective electrode on which a protective coating was made from cellulose acetate is, however, disadvantageous in that it takes a long time to activate and stabilize the electrode, its response time is very slow, ranging from minutes to tens of minutes, and the poor adhesion of cellulose acetate to the electrode surface deteriorates the sensitivity and lifetime of the electrode.
With this background in mind, the present inventors have intensively and thoroughly conducted research on solid-state membrane-based electrode and found that the use of hydrophilic polyurethane as protective membranes of potentiometric sensors resulted in a fast response time and a great improvement in selectivity for chloride, fast electrode activation, and biocompatibility.
Therefore, it is the object of the present invention to provide chloride-selective electrodes, which provide excellent selectivity for chloride, fast activation and response time, and superior biocompatibility.
In accordance with the present invention, the above and other objects of the present invention could be accomplished by a provision of a solid-state membrane-based chloride-selective electrode, comprising an insoluble metal layer and a protecting membrane formed of hydrophilic polyurethane.